Photothermographic elements of the type wherein photographic images are formed through heat development are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic elements generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a reducing agent for silver, and a toner for controlling the tone of silver, typically dispersed in a binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
These photothermographic elements are widely used in the general photography, microphotography, medical image recording, graphic printing plate, and other fields. The light sources used in combination therewith have a wide range of wavelength including ultraviolet (UV), visible and near infrared light. With the recent advance of lasers and light-emitting diodes, the application of photothermographic elements wherein exposure is carried out at an oscillation wavelength from visible light to the near infrared region is increasing.
Also developed recently is a recording system using a thermal head which is simple and inexpensive as compared with the laser recording apparatus. The recording elements for this application are the same as the above-described thermographic recording elements except that photosensitive silver halide serving as the photocatalyst is omitted.
Regarding thermographic recording elements capable of forming high contrast images suitable for the graphic printing plate application, a number of patents are known. These patents disclose hydrazine derivatives, acrylonitrile derivatives, isoxazolone derivatives, tetrazolium derivatives, etc. as the contrast enhancer for producing high contrast images. For example, U.S. Pat. Nos. 5,464,738, 5,496,695, 5,512,411, 5,536,622, Japanese Patent Application Nos. 228627/1995, 215822/1996, 130842/1996, 148113/1996, 156378/1996, 148111/1996, and 148116/1996 describe hydrazine derivatives. Japanese Patent Application No. 83566/1996 describes compounds having quaternary nitrogen. U.S. Pat. No. 5,545,515 describes acrylonitriles. Illustrative examples of these compounds are Compounds 1 to 10 in U.S. Pat. No. 5,464,738, Compounds H-1 to H-28 in U.S. Pat. No. 5,496,695, Compounds I-1 to I-86 in JP Appln. No. 215822/1996, Compounds H-1 to H-62 in JP Appln. No. 130842/1996, Compounds I-1 to I-21 in JP Appln. No. 148113/1996, Compounds 1 to 50 in JP Appln. No. 148111/1996, Compounds 1 to 40 in JP Appln. No. 148116/1996, Compounds P-1 to P-26 and T-1 to T-18 in JP Appln. No. 83566/1996, and Compounds CN-1 to CN-13 in U.S. Pat. No. 5,545,515.
One of the serious problems associated with thermographic recording elements is a high developing temperature which can cause film deformation, blackened density variations, evolution of odorous or stimulative gases, and volatilization of acidic substances. The vapor acidic substances will stick to the surrounding electronic apparatus, causing corrosion. It is thus desired to lower the heat development temperature. Higher temperatures are required for heat development particularly when a water-dispersed polymer latex is used as the binder or when high contrast images are produced using ultrahigh contrast enhancers. In these cases, it is a very strong desire to lower the heat development temperature.